Jeyaretnam was born in 1959 in Singapore. He is the eldest child of J. B. Jeyaretnam (sometimes known as "JBJ") and Margaret Cynthia Walker. His parents met in the United Kingdom in 1950 while studying law at University College London. J. B. Jeyaretnam, a barrister, became one of Singapore's most well-known opposition politicians and democracy advocates and was the first opposition member ever to be elected to Singapore's Parliament in 1981. Margaret Walker was a lawyer originally from England was later one of the first women lawyers in Singapore and Registrar of the Diocese of Singapore and Malaysia. Jeyaretnam was followed by a brother who died at birth, and then by Philip Jeyaretnam who was born in 1964. Jeyaretnam is now married to Amanda Jeyaretnam, with a 17-year-old son, Jared[citation needed].

Upon graduation, Jeyaretnam returned to Singapore where he took up his first employment with Wardley (the merchant banking arm of HSBC in the 1980s), working as an Assistant Manager in the Lending Department. In 1984 Wardley decided to transfer him overseas and he was sent to work in their Hong Kong branch. This move by his employer coincided with the time of his father's second election victory in 1984. Whilst declining to join his father's Workers' Party (WP), he contributed by writing sections of the party's manifesto dealing with economic policy and articles for the party newsletter The Hammer.

From 1987 to the early-1990s, Jeyaretnam worked in London and Tokyo for Continental Bank, Banque Indosuez and Lehman Brothers. He then moved to Nomura International in London as head of Japanese Warrants and later as Proprietary Trader. He met his wife-to-be Amanda while working there. They married in 1995 and had a son in 1997.

From 1998, Jeyaretnam moved on from the Asian derivatives markets to become a hedge fund manager specialising in event–driven investing. He set up and managed his own Funds from 2004-08. During this time in London, he was a member of the Singapore UK Association (SUKA) and later served on the association's committee working with fellow expat Singaporeans[citation needed].

Jeyaretnam lacked prominence in Singapore until the media attention after his father's death. Jeyaretnam's eulogy at his father's funeral was broadcast over YouTube and other new media sites. Subsequently, he arranged a memorial event and exhibition, working closely on the project with his father's supporters and civil rights groups.

In 2007, J. B. Jeyaretnam formed the Reform Party. The survival of the new opposition party seemed to have been in question when he suffered a heart attack and died on 30 September 2008.[3] With the demise of his father, Jeyaretnam began an active role in Singaporean politics. He started contributing articles on economic themes to the Singaporean online blog, The Online Citizen. He gave his first public "political" speech alongside other prominent opposition politicians at a memorial event held in memory of his father in Hong Lim Park on New Year's Eve 2008. It was here that Jeyaretnam spoke of his belief that it was possible for Singapore to have both prosperity and liberty. During this period he also started working with prominent non-partisan groups in their campaign to have a chair elected in his father's name and in establishing a foundation for study which will sponsor Singaporean students in human rights law and politics.

On 10 April 2009 it was announced in The Straits Times and other sources that Kenneth Jeyaretnam had joined the Reform Party. Eugene Tan, a law lecturer at Singapore Management University, was quoted as saying: "Mr Jeyaretnam has given the Reform Party a tremendous boost given his academic achievements, professional background and political pedigree". Before long, Jeyaretnam was co-opted into the party's Central Executive Committee (CEC) and voted into the vacant position of Secretary-General.

Jeyaretnam in a February 2010 statement available on the Reform Party's website indicated that "The Reform Party would go much further though in dismantling or privatizing the whole GLC (Government-linked companies) structure starting with the privatization of GIC (Government of Singapore Investment Corporation) and Temasek (Temasek Holdings) and giving Singaporean citizens a direct stake, whether through shares, deferred shares or quasi-equity, in their assets. We would seek to sell off or break-up most of the GLCs which control such a substantial portion of economic activity (up to 60% according to some estimates) which has in our view a detrimental effect on the growth of a vibrant private sector in Singapore."[citation needed]

Jeyaretnam speaking in the rain at a Reform Party rally at Speakers' Corner on 15 January 2011

In his maiden election speech in late April 2011, Kenneth Jeyaretnam said the objective of the Reform Party is to win a few seats in the May 2011 general election, adding that competition in politics will lead to better and more intelligent policies. He also sounded a warning that the ruling People's Action Party (PAP) will likely open the floodgates to more foreigners to enter Singapore again once they form the government. "Already you can see that. The Reform Party was the first to talk about how the government's track record was hollow. They always talk about the high rate of economic growth, but the economic growth is created just by bringing in cheap foreign labour. It is not created by raising the incomes of Singaporeans," he said.

Jeyaretnam criticized the PAP for failing to improve the lives of ordinary Singaporeans as their median incomes have remained stagnant over the last few years, attributing this to the PAP's open-door policies to immigrants. "If at this election you don't take action now, you will expect the floodgates to open again," he warned.